1. Field of the Invention
This invention relates to improvements in a dynamic pressure type fluid bearing device wherein the load capacity during rotation is maintained constant.
2. Description of the Prior Art
The dynamic pressure type fluid bearing device is a bearing device in which a rotational member is mounted for rotation relative to a fixed member and a force for maintaining the positional relation of the rotational member to the fixed member in a predetermined condition is generated on the basis of the rotation of the rotational member. To maintain the two members in a predetermined positional relation, the dynamic pressure must be made constant. For this purpose, an outwardly communicating passage, namely, a circulation path for lubricant, is formed in a pressure chamber and this circulation path is adapted to be opened by the rotational member floating upwardly. However, in the prior art, there has been a problem in the position whereat the circulation path is formed and therefore, it has been difficult to say that the pressure control is effected reliably. These circumstances will hereinafter be described in greater detail by reference to FIG. 1 of the accompanying drawings.
In FIG. 1, a cylindrical hole 2 formed in a fixed housing 1 has a cylindrical internal bearing surface 3 for radial load and a planar first bearing surface 4 for thrust load while, on the other hand, a shaft 5 rotatably fitted in the cylindrical hole 2 has a cylindrical outer bearing surface 7 for radial load having dynamic pressure generating grooves 6 and a convex conical second bearing surface 8 for thrust load. The housing 1 is provided with a circulation path 12 leading from a pressure chamber 11 defined by the bearing surfaces 4 and 8 for thrust load to the upper surface of the housing 1. This circulation path 12 is formed so as to open near the boundary 13 between the bearing surfaces 7 and 8 when the shaft 5 is stationary.
In this prior art, when the shaft 5 rotates, the lubricant between the bearing surfaces 7 and 3 flows into the pressure chamber 11 due to the pumping action of the grooves 6 and the pressure in the pressure chamber 11 is increased, so that the shaft 5 moves upwardly. By the upward movement of the shaft 5, the portion below the boundary 13 becomes opposed to the circulation path 12 and the lubricant in the pressure chamber 11 flows out to the outer peripheral surface of the shaft 5 through the circulation path 12, whereby the amount the shaft 5 floats upwardly is kept substantially constant.
Now, where such a bearing is used in a flat motor or the like, a rotor fixed to the shaft 5 and a stator fixed to the housing 1 are opposed to each other through an axially minute clearance and therefore, it is desired to minimize the relative displacement of the rotor and the stator. However, it is technically difficult to accurately provide the circulation path 12 near the boundary 13 and therefore, it is very difficult to minimize the amount of float-up of the shaft 5 floats upwardly, namely, the relative displacement of the rotor and the stator.
Also, the suction force acting between the rotor and the stator acts as a thrust load and the magnitude thereof is considerably great. However, when the shaft 5 is stationary, the top of the bearing surface 8 makes point contact with the bearing surface 4 of the housing 1 and therefore, the contact surface pressure becomes so high that an impression will sometimes be created in the bearing surface 4.